1. Field of the Invention
The disclosures herein relate to an image forming apparatus having a developing device configured to develop an image by attaching a toner hopping on a surface of a toner carrier to a latent image formed on a latent image carrier.
2. Description of the Related Art
Japanese Patent Application Publication No. 2010-60918 (hereinafter called “Patent Document 1”) discloses an image forming apparatus of this kind. The developing device of the image forming apparatus disclosed in Patent Document 1 includes a rotatable cylindrical toner carrier roller. The cylindrical toner carrier roller includes long and slim electrodes elongated in a rotational axis direction arranged at predetermined pitches. Alternating electric fields are formed between the adjacently arranged long and slim electrodes on the surface of the cylindrical toner carrier roller. The toner may hop from directly above one of the adjacently arranged electrodes to the other, or vice versa based on directional changes of the alternating electric fields. The toner is then carried to a developing region facing the latent image carrier with a rotational movement of the toner carrier roller while reciprocally hopping between the adjacent electrodes. In the developing region, the toner hops from the surface of the toner carrier roller toward and neat to the latent image carrier and then is attracted by the electric fields formed by the electrostatic latent image. The toner attracted to the electrostatic latent image is attached to the latent image accordingly. The latent image is thus developed by the attachment of the toner.
An image forming apparatus of another kind is also generally known in the art. In such image forming apparatus, the toner is not simply carried by a surface movement of the toner carrier roller while reciprocally hopping between the electrodes but the hopping toner on the surface of toner carrier roller itself moves in a predetermined direction by the hopping behavior. For example, in the image forming apparatus having the toner carrier roller on which three electrodes A, B and C phases are repeatedly arranged in this order, the toner is transferred toward the developing region by allowing the toner to sequentially hop from the A phase electrode to the B phase electrode, the B phase electrode to the C phase electrode, the C phase electrode to the A phase electrode, and the like on the surface of the toner carrier roller.
With these kinds of image forming apparatuses utilizing the hopping toner for the development (hereinafter called a “hopping type developing system”), the low voltage development may be achieved to an extent which may not be achieved by the conventional one-component developing system or two-component developing system. For example, the toner may be selectively attached to the electrostatic latent image having a potential difference of several dozen fractional V between the latent image formed region and a peripheral non-image forming region.
However, if the electrostatic charge of the toner on the surface of the toner carrier roller is lowered, the toner exhibits insufficient hopping on the surface of the toner carrier roller, which may result in development degradation.
The image forming apparatus disclosed in Patent Document 1 is provided with a cleaner device, and the toner exhibiting inferior hopping behavior due to the lowered the electrostatic charge is caused to hop from the surface of the toner carrier roller onto the photoreceptor surface such that the toner is discharged from the surface of the toner carrier roller, and the cleaner device collects the discharged toner from the photoreceptor surface. After the toner is removed from the toner carrier roller, new toner having appropriate electrostatic charge is supplied on the surface of the toner carrier roller. Accordingly, the latent image is developed by attaching the toner exhibiting excellent hopping behavior on the toner carrier roller to a latent image formed on the photoreceptor. Thus, the occurrence of the development degradation may be reduced.
In the image forming apparatus disclosed in Patent Document 1, when the toner having a low electrostatic charge is discharged from the surface of the toner carrier roller, the toner is caused to hop from the surface of the toner carrier roller to the photoreceptor surface by applying alternating voltages having an amplitude greater than that for the developing process and having differing phases to the adjacent first and second electrodes while the toner is removed by the cleaner device. Accordingly, when the alternating voltages having the amplitude relatively greater than that for the developing process are applied to the first and the second electrodes of the toner carrier roller, the toner having the lowered electrostatic charge may hop to the photoreceptor to some extent.
However, if the potential difference between the first electrode and the second electrode is too large, the insulator between the first and the second electrodes may be broken due to current leakage having occurred between the first and the second electrodes. Further, if the alternating voltages having the potential difference that may induce no insulator breakage are applied to the first and the second electrodes while the surface of the toner carrier roller is being cleaned, the electric fields sufficiently large to cause the toner having the drastically lowered electrostatic charge to hop from the toner carrier roller to the photoreceptor may not be generated. In this case, the toner having the drastically lowered electrostatic charge may remain on the surface of the toner carrier roller without hopping from the toner carrier roller to the photoreceptor.